DESCRIPTION (Investigator's Abstract): Elucidating the developmental events that lead to the differentiation of a multitude of very specialized neurons is an important goal for understanding normal nervous system development, for explaining and intervening in congenital birth defects, and for designing neuronal replacement therapies for degenerative CNS diseases. The goals of this research program are to elucidate determinative events: 1) at the very beginning of an embryonic cell's developmental pathway to becoming a neuron; and 2) at the terminal division of an neuroepithelial cell in a neuronal lineage. Although the first morphological indication of the nervous system, the neural plate, is induced by the underlying chordamesoderm during gastrulation, this induction is not the first step in the developmental pathway leading to the differentiation of neurons. We and others have shown that interactions prior to neural induction bias the ability of blastomeres to become neural progenitors. Some blastomeres are committed to neuronal lineages prior to gastrulation, and this commitment depends upon the products of localized maternal RNAs (Hainski and Moody, 1992). We now propose experiments to elucidate the translational regulation of these RNAs during cleavage stages (Specific Aim 1) and to identify their nucleic acid sequence(s) (Specific Aim 2). Specific Aim 3 pursues a new line of investigation of fate determination at a neuron's terminal division. Neurons start their differentiation program with a terminal mitosis, and several lines of evidence suggest that the genetic programs which determine the kind of neuronal cell to be produced are active around the time of the terminal division. In an effort to define the molecular regulation of the choice to become a neuron at the terminal division, we have studied the developmental expression of a neuron-specific beta-tubulin protein that is first detected around the time of terminal mitosis in avians and mammals. During the past year, we have isolated, cloned and sequenced a large part of the 5' untranslated region of the rat mRNA for Class III beta- tubulin. We now propose to completely sequence this region and the promoter region of the gene, and identify the elements in these regions that confer neuronal specificity. This information will allow us, in future experiments, to identify the trans-acting factors that regulate neuron-specific expression, and thus neuronal identity.